MFC: r291035
The problem report was for a crash that happened when smbfs was
trying to do a mount. Given the backtrace,
it appears that the crash occurred when smb_vc_create() failed and then
called smb_vc_put() with vcp->vc_iod == NULL. smb_vc_put() subsequently
called smb_vc_disconnect() with vcp->vc_iod == NULL, causing the crash.
This patch adds a check for vcp->vc_iod != NULL in smb_vc_disconnect() to
avoid the crash. It also fixes the case in smb_vc_create() where
kproc_create() fails so that it destroys the mutexes and sets
vcp->vc_iod == NULL before free()'ing the iod structure.

MFC: r290959
When the smbfs iod thread (smb_iod_thread()) is shutting down, smb_iod_destroy()
would call smb_iod_request(). This call could return as soon as the
wakeup(evp) in smb_iod_main() call is done and then could destroy
the mutexes. This caused a race with the rest of smb_iod_main()s
use of these mutexes.
A crash reported on freebsd-stable@ by Christian Kratzer was
diagnosed as a use of one of these mutexes after it was destroyed.
This patch moves destruction of the mutexes from smb_iod_destroy()
to the end of smb_iod_thread(), so that they aren't destroyed before
the thread is done with them. Christian comfirmed that the patch
stopped the crashes from happening.

MFC r264494:
Use SMB_QUERY_FS_SIZE_INFO request to populate statfs structure.
When server doesn't support this request, try to use SMB_INFO_ALLOCATION.
And use SMB_COM_QUERY_INFORMATION_DISK request as fallback.

MFC r264600:
Remove redundant unlock.

This code was removed from the opensolaris and darwin's
netsmb implementations, in DfBSD it also has been disabled.

MFC r264494:
Use SMB_QUERY_FS_SIZE_INFO request to populate statfs structure.
When server doesn't support this request, try to use SMB_INFO_ALLOCATION.
And use SMB_COM_QUERY_INFORMATION_DISK request as fallback.

MFC r264600:
Remove redundant unlock.

This code was removed from the opensolaris and darwin's
netsmb implementations, in DfBSD it also has been disabled.